Objective lens for high-density optical focusing and an optical disk in an optical pickup

ABSTRACT

An optical pickup includes a light source emitting a laser beam and an optical path changing unit altering a traveling path of an incident beam. An objective lens, disposed on an optical path between the optical path changing unit and an optical disk, focuses the incident beam from the light source to form a light spot on the optical disk of the objective lens. The optical pickup further includes a photodetector and an detecting-correcting unit, arranged on the optical path between the optical path changing unit and the objective lens, performing at least one of detecting the thickness of the optical disk and correcting aberration caused by thickness variations of the optical disk. The objective lens includes a first transmitting portion divergently transmitting an incident beam, where the first transmitting portion is at a relatively near-axis region from an optical axis of the objective lens. A second transmitting portion transmits the incident beam, where the second transmitting portion is arranged facing the first transmitting portion. A first reflecting portion condenses and reflects the incident beam from the first transmitting portion, where the first reflecting portion is formed around the second transmitting portion. A second reflecting portion condenses and reflects the incident beam from the first reflecting portion towards the second transmitting portion, where the second reflecting portion is formed around the first transmitting portion.

This application is a divisional of U.S. patent application Ser. No.09/725,879, filed Nov. 30, 2000, now U.S. Pat. No. 6,938,890, thedisclosure of which is incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an objective lens having a highnumerical aperture (NA) for high-density optical focusing, an opticalpickup adopting the objective lens, and a relatively thin optical disksuitable for the optical pickup.

2. Description of the Related Art

The information recording and reproduction density in an optical diskincreases as the size of a light spot focused on the optical disk by anoptical pickup decreases. In general, the size of the light spot focusedon an optical disk is proportional to a wavelength (λ) of a light sourceover a numerical aperture (NA) of an objective lens. Thus, as shown in afunctional relationship (1) below, the shorter the λ of the light sourceand the larger the NA of the objective lens, the smaller the size of thelight spot.size of light spot ∝ λ/NA  (1)

For a higher recording and reproduction density, an optical pickup needsa light source capable of emitting a shorter wavelength of light and anobjective lens having a high NA. However, due to the limitation inmanufacturing a single objective lens, it is impossible to manufacturean objective lens having an NA of 0.8 or higher, making it difficult tosatisfy the need for an allowable error below an optical aberration of0.07λ_(ms). A conventional optical pickup for information recording andreproduction and objective lens is illustrated in FIGS. 1 and 2.

The conventional optical pickup illustrated is capable of recodinginformation with high density over an optical disk 1 having a 0.1-mmthick protective layer. The optical pickup includes a light source 11having a wavelength, λ, of 400 nm, a grating 19 diffracting andtransmitting an incident beam, a first polarization beam splitter (PBS)21 altering the traveling path of the incident beam in a predeterminedpolarization direction, a λ/4 plate 23 guiding a circular polarized beamto the optical disk 1, and an objective lens unit 50 having an NA of0.85. The optical pickup further includes a second PBS 27 transmittingor reflecting the incident beam from the optical disk 1 and,subsequently, from the first PBS 21. A main photodetector 31 receivesthe incident beam passed through the second PBS 27 and detects aninformation signal from the incident beam. A servo photodetector 37receives the beam reflected from the second PBS 27 and detects an errorsignal therefrom.

The optical pickup further includes a collimating lens 13 collimatingthe incident beam, a beam shaping prism 15 shaping the incident beam,and a λ/2 plate 17 delaying the phase of the incident beam. Thecollimating lens 13, the beam shaping prism 15, and the λ/2 plate 17 arearranged on the optical path between the light source 11 and the grating19. A second λ/2 plate 25 delaying the phase of the incident beam isfurther disposed on the optical path between the first PBS 21 and thesecond PBS 17. A first condensing lens 29 condenses the incidentparallel beam and it is arranged between the second PBS 27 and the mainphotodetector 31. A second condensing lens 33 condenses the incidentparallel beam and an astigmatism lens 35 creates astigmatism. The secondcondensing lens 13 and the astigmatism lens 13 are arranged between thesecond PBS 27 and the servo photodetector 37. A monitoring photodetector41 monitors the optical power of the light source 11 from the beamreflected by the first PBS 21 and condensed by a third condensing lens39. The objective lens unit 50 includes an objective lens 51 focusingthe incident beam and a semi-spherical lens 55, which is arrangedbetween the objective lens 51 and the optical disk 1, to increase the NAof the objective lens unit 50. In the above configuration of theobjective lens unit 50, an NA of 0.6 can be secured by the objectivelens 51 and increased by the semispherical lens 55.

Referring to FIG. 2, as long as the semispherical lens 55 does not causean incident beam to refract, the NA of the semispherical lens 55 isproportional to the product of sin θ and a refractive index, n, of thesemispherical lens 55, wherein θ is the maximum incident angle θ oflight into the semispherical lens 55, which is expressed by equation(2). Thus, the NA of the objective lens unit 50 can be increased up to0.85.NA=n sin θ  (2)

However, the conventional optical pickup includes two lenses to achievesuch high NA. Thus, if a tilting occurs between the semispherical lens55 and the objective lens 51, keeping a low optical aberration becomesdifficult. When the semi-spherical lens 55 and the objective lens 51 areassembled into the objective lens unit 50, a restrictive control ofdistance and tilting error between the semispherical lens 50 and theobjective lens 51 is needed, thereby making mass production of theobjective lens unit 50 difficult.

In manufacturing the optical disk 1, an error in thickness is 3% ormore. Accordingly, if the optical disk 1 has a thickness of 0.1-mm, thethickness error is ±3 μm or more. Such thickness error creates seriouscoma aberration and astigmatism when the objective lens unit 50 has anNA of 0.8 or more. Thus, a restrictive error management is desired inmanufacturing a 0.1 mm thick optical disk so that the thickness error iswithin ±3 μm. However, it is difficult to produce a 0.1 mm thick opticaldisk with ±3 μm thickness error, or with a maximum thickness error of 5μm, on a large scale.

In the conventional optical pickup described above, aberration caused byan error in thickness of the optical disk 1 is corrected by adjustingdistance between the objective lens 51 and the semispherical lens 55.However, the configuration of an actuator for adjusting the distancebetween the objective lens 51 and the semispherical lens 55 iscomplicated.

SUMMARY OF THE INVENTION

Various objects and advantages of the invention will be set forth inpart in the description which follows and, in part, will be obvious fromthe description, or may be learned by practice of the invention.

The present invention is achieved by providing an objective lens forhigh-density focusing. The objective lens is a single lens having a highnumerical aperture (NA). The objective lens is included in an opticalpickup that also includes a relatively thin optical disk.

According to an aspect of the present invention, an objective lens isprovided including a first transmitting portion divergently transmittingan incident beam, where the first transmitting portion is at arelatively near-axis region from an optical axis of the objective lens,and a second transmitting portion transmitting the incident beam, wherethe second transmitting portion is arranged facing the firsttransmitting portion. A first reflecting portion, having a negativepower, condenses and reflects the incident beam from the firsttransmitting portion, wherein the first reflecting portion is formedaround the second transmitting portion. A second reflecting portion,having a positive power, condenses and reflects the incident beam fromthe first reflecting portion towards the second transmitting portion,wherein the second reflecting portion is formed around the firsttransmitting portion.

According to another aspect of the present invention an optical pickupincludes a light source emitting a laser beam, an optical path changingunit altering a traveling path of an incident beam, an objective lens,disposed on an optical path between the optical path changing unit andan optical disk, focusing the incident beam from the light source toform a light spot on the optical disk, and a photodetector receiving thebeam reflected from the optical disk and passed through the objectivelens and the optical path changing unit. Further, a detection-correctionunit, arranged on the optical path between the optical path changingunit and the objective lens, performs at least one of detecting thethickness of the optical disk and correcting aberration caused bythickness variations of the optical disk.

In another embodiment, an optical disk is provided including aninformation substrate having an incident surface receiving light torecord and reproduce information, and a recording surface on which aninformation signal is recorded and from which at least a portion of anincident beam is reflected, wherein the thickness from the incidentsurface of the information substrate to the recording surface is lessthan 0.1-mm.

These together with other objects and advantages which will besubsequently apparent, reside in the details of construction andoperation as more fully hereinafter described and claimed, referencebeing had to the accompanying drawings forming a part hereof, whereinlike numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objectives and advantages of the present inventionwill become more apparent by describing in detail preferred embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a diagram illustrating an optical arrangement of aconventional optical pickup including an objective lens unit forhigh-density focusing;

FIG. 2 is a diagram illustrating the objective lens unit in FIG. 1having a high numerical aperture (NA);

FIG. 3 is a diagram illustrating a configuration of an objective lensfor high-density focusing for a parallel incident beam, in accordancewith the present invention;

FIG. 4 is a diagram illustrating intensity distribution of a light spotfocused by an objective lens for a parallel incident beam, in accordancewith the present invention;

FIG. 5 is a diagram illustrating an alternative embodiment of theobjective lens in FIG. 3;

FIG. 6 is a diagram illustrating the optical arrangement of an opticalpickup including an objective lens for high-density focusing, inaccordance with the present invention; and

FIG. 7 illustrates an alternative embodiment of the optical pickup, inaccordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 3, an objective lens 150 for high-density focusingincludes a first transmitting portion 151 located in a near axis regionaround the optical axis for divergently transmitting an incident beam. Asecond transmitting portion 153, facing the first transmitting portion151, transmits the incident beam. A first reflecting portion 155,arranged around the second transmitting portion 153, condenses andreflects the incident beam which has passed through the firsttransmitting portion 151. A second reflecting portion 157, arrangedaround the first transmitting portion 151, condenses and reflects theincident beam reflected from the first reflecting portion 155 towardsthe second transmitting portion 153.

In an exemplary embodiment, the first transmitting portion 151 has aconcave curvature with a negative power to remove optical fieldaberration. Specifically, the first transmitting portion 151 is designedwith spherical and/or aspherical surfaces for minimum optical aberrationaccording to the condition of the incident beam. The first reflectingportion 155 has a concave reflecting surface with a negative power,which maintains a high NA of 0.7 or more, and condenses and reflects theincident beam so that the size of the second transmitting portion 153can be minimized. As a result, a light spot with ultra high resolutionmay be focused on an optical disk 100 for information recording andreproduction. The second reflecting portion 157 has a concave reflectingsurface with a positive power for minimizing spherical and comaaberration and other staying optical aberration of the incident beam. Inthe alternative, the second reflecting portion 157 may be designed withan aspherical surface.

As shown in FIG. 3, the second transmitting portion 153 is designed tobe planar. Alternatively, the second transmitting portion 153 may bedesigned with the same curvature as that of the concave reflectingsurface of the first reflecting portion 155 allowing an easiermanufacturing process. The space enclosed by the first transmittingportion 151, the second reflecting portion 157, the second transmittingportion 153, and the first reflecting portion 155 is filled with anoptical material having a refractive index, n, different from that ofair. Thus, the objective lens 150 may be obtained by processing atransparent optical material into the configuration described above. Theobjective lens 150 is used to focus a parallel incident beam on theoptical disk 100.

The optical disk 100 may have a thickness D of 0.2 mm or less or ofabout 0.05 mm (50 μm), for instance, to overcome coma aberration andastigmatism which typically occur with objective lenses having a highNA. Furthermore, the optical disk 100 includes an information substrate101 having an incident surface 103 for receiving light to record andreproduce information, a protective layer 105, and a recording surface107 on which an information signal is recorded and from which at least aportion of the incident beam is reflected. The thickness D of theoptical disk 100 corresponds to the thickness of the protective layer105, i.e., from the incident surface 103 to the recording surface 107 ofthe information substrate 101.

Typically, the incident beam cannot be used to record information on orreproduce information from the optical disk 100 because the beam passingfrom the first transmitting portion 151 through the second transmittingportion 153 diverges, which inhibits focusing on a recording surface 107of the optical disk 100. Also, the amount of light reflected from therecording surface 107 of the optical disk 100 is significantly reduced.Thus, the second transmitting portion 153 serves as a shield withrespect to the light incident directly from the first transmittingportion 151, i.e., light of a near-axis beam. Only the beam incidentfrom the first transmitting portion 151 toward the first reflectingportion 155, which is then reflected towards the second reflectingportion 157 and, subsequently, incident on the second transmittingportion 153, is used to record information on or reproduce informationfrom the optical disk 100.

When the near-axis beam is shielded by the second transmitting portion153, the size of the light spot focused on the recording surface 107 ofthe optical disk 100 is sharply reduced. Unnecessary side lobecomponents, as indicated by “s” in FIG. 4, to be later described, appeararound the light spot corresponding to the size of the secondtransmitting portion 153. The side lobe components degrade theresolution of high-density light spot.

Further, if the first reflecting portion 155 is designed to have aconcave curvature as the objective lens 150, the beam reflected by thefirst and second reflecting portions 155 and 157 and incident toward thesecond transmitting portion 153, may be focused in a small region. Inthis case, the second transmitting portion 153 may be formed to have adiameter much smaller than the outer diameter of the first reflectingportion 155 and smaller than the outer diameter of the beam incident onthe first reflecting portion 155, thereby significantly reducing theside lobe components of the light spot. The objective lens 150,according to the present invention, may be manufactured as a single lensconfiguration with an NA of 0.8 or higher, which enables to form ahigh-density light spot with ultra high resolution.

In the present embodiment, when a ratio of the outer diameter of thesecond transmitting portion 153 to the outer diameter of the beamincident on the first reflecting portion 155 is 0.5 or less or, when theouter diameter of the second transmitting portion 153 and the outerdiameter of the beam incident on the first reflecting portion 155satisfy condition (3) below, the objective lens 150 can be effectivelyused to form a small light spot for reproducing information from theoptical disk 100.0.1<diameter of second transmitting portion/outer diameter of lightincident on first reflecting portion<0.3  (3)

Furthermore, when an angle, α, between the optical axis and an outermostray of the incident beam, which passes through the first transmittingportion 151 and reflects on the first and second reflecting portions 155and 157 and thereafter passes through the second transmitting portion153, is greater than or equal to 36° or satisfies a condition (4) below,the objective lens 150 effectively minimizes the size of the light spotfocused on the optical disk 100.36°≦α≦65°  (4)

An example of the optical data for the objective lens 150 having theabove configuration according to the present invention is illustrated inTable 1. Table 1 shows the design data of the objective lens 150suitable for a parallel incident beam when a working distance, d,between the emitting surface of the objective lens 150 and the receivingsurface 103 of the optical disk 100 is 1.1 mm. The optical disk 100 ismade of conventional material used in the optical field. Table 2 showsthe aspherical coefficients of the aspherical surfaces listed in Table1.

TABLE 1 Radius of Thickness or curvature (mm) Distance (mm) Medium Firsttransmitting −0.59998 2.700000 BACD5_HOYA portion (aspherical surface 1)First reflecting −14.44606 −3.100000 BACD5_HOYA portion Secondreflecting 5.39477 3.100000 BACD5_HOYA portion (aspherical surface 2)Second transmitting ∞ 0.100000 AIR portion Optical Disk ∞ 0.100000 —

TABLE 2 Aspherical coefficient K A B C D Aspherical surface 1 −0.209233  0.137213E+00   0.3288285E+00 −0.409641E+00   0.292448E+01 Asphericalsurface 2 −0.164077 −0.415232E−03  −0.295529E−04   0.208258E−05−0.760111E−06

When a light spot is focused by the objective lens 150 designed with theabove data, the intensity of the light spot distributes as illustratedin FIG. 4. The side lobe components indicated by “s” are maintained at5% or less of the peak intensity of the light spot. At an intensitylevel of 1/e², where e is energy, the size of the light spot is 0.35 μmin the tangential direction of the optical disk 100 and 0.37 μm in theradial direction of the optical disk 100. Thus, the light spot appearsas a miniature light spot almost close to a circle.

Therefore, the objective lens 150 can achieve a high NA of 0.85 or morewith a single lens configuration and thus, it may be applied tominiature optical systems that need a high NA. For instance, theobjective lens 150 may be applied to microscopes equipped with a chargecoupled device (CCD) camera that includes an objective lens and anocular lens, optical exposure apparatuses for mask patterning in themanufacturing of semiconductor devices that include an objective lens, alight source and a collimating lens, and mastering apparatuses tomanufacture optical disks. A person of ordinary skill in the art willappreciate that the optical design data for the objective lens 150 canbe varied for a condensing or diverging incident beam.

FIG. 5 illustrates an alternative embodiment or configuration of theobjective lens 150 in FIG. 3 in accordance with the present invention.The configuration of the objective lens 150 illustrated in FIG. 5includes a path difference generating portion 157 a that projects fromor recesses into the concave curvature of the second reflecting portion157. The path difference generating portion 157 a generates a differentpath for at least a portion of the beam incident on the secondreflecting portion 157 such that the side lobe components on the lightspot are reduced.

In this configuration, a difference in optical paths occurs, i.e., aphase difference, between the beam incident on the path differencegenerating portion 157 a from the first reflecting portion 155 and thebeam incident on the other portion of the second reflecting portion 157.Interference caused by the different optical paths can further reducethe side lobe components of the light spot. Alternatively, the pathdifference generating portion 157 a may be formed in the firstreflecting portion 155.

The optical disk 100, in accordance to the present invention, may have athickness D less than 0.1 mm less, such that coma aberration andastigmatism, which occur at the objective lens 150 having a high NA, maybe overcome without the need to correct the spherical aberration as aresult of thickness variations of the optical disk. For exemplarypurposes, the optical disk 100 has a thickness D less than 50 μm.Further, the optical disk 100 can be manufactured within a thicknesserror range of ±5 μm with a maximum thickness error of 5 μm. Thus,correcting for spherical aberration due to thickness variations of theoptical disk may be omitted. The optical disk 100 further includes theinformation substrate 101 described in FIG. 3.

FIG. 6 illustrates an optical arrangement of an optical pickup adoptingthe objective lens for high-density focusing in accordance with thepresent invention. The optical pickup includes a light source110emitting a laser beam. In an exemplary embodiment, the light source110 is a semiconductor laser, such as an edge emitting laser, emitting ablue laser beam having a wavelength of 500 nm or less. The laser beamemitted from the light source 110 is collimated by a collimating lens125 and passed as an incident beam to an optical path changing unit 120.The optical path changing unit 120 alters the traveling path of theincident beam. The optical pickup further includes an objective lens 150focusing the incident beam to form a light spot on an optical disk 100and a photodetector 160. The photodetector 160 receives the beamreflected from the optical disk 100 and passed through the objectivelens 150 and the optical path changing unit 120 to detect informationand error signals. In the present embodiment, the optical disk 100 has aprotective layer having a thickness of 0.2 mm or less, for instance 0.05mm, for a recording density of, for instance, about 20 gigabytes ormore.

The optical path changing mean 120 includes a polarization beam splitter(PBS) 121 having a mirror surface 121 a to transmit and reflect theincident beam according to the polarization of the incident beam, and awave plate 125 to change the polarization of the incident beam. The PBS121 serves to shape the incident beam from the light source 110. When avertical cavity surface emitting layer (VCSEL) for emitting light in thestack of semiconductor material layers is adopted as the light source110, a cubic type PBS is employed in the optical pickup.

The PBS 121 arranged on the optical path between the light source 110and the objective lens 150 transmits one polarization component of theincident beam and reflects the other polarization component of theincident beam. In an exemplary embodiment, the wave plate 125, arrangedon the optical path between the PBS 121 and the objective lens 150, is aquarter-wave plate phase shifting by λ/4 the beam emitted from the lightsource 110. When light travels back through the objective lens 150 afterbeen reflected from the optical disk 100, the wave plate 125 changes thepolarization of the incident beam such that the beam reflected from theoptical disk 100 is reflected by the mirror surface 121 a of the PBS 121and goes towards the photodetector 160.

The objective lens 150 is driven in the focusing and tracking directionof the optical disk 100 by an actuator 159. As previously mentioned, theobjective lens 150 includes a first transmitting portion 151 divergentlytransmitting the incident beam and a second transmitting portion 153arranged facing the first transmitting portion 151. A first reflectingportion 155 arranged around the second transmitting portion 153,condenses and reflects the beam incident through the first transmittingportion 151. A second reflecting portion 157 is arranged around thefirst transmitting portion 151 to condense and reflect the incident beamreflected from the first reflecting portion 155 around the secondtransmitting portion 153. In the present embodiment, the first andsecond transmitting portions 151 and 153 and the first and secondreflecting portions 155 and 157 have approximately the sameconfigurations and functions as those illustrated previously, and thus adescription thereof will not be repeated.

The photodetector 160 receives the incident beam reflected from theoptical disk 100 and directed toward the photodetector 160 by the PBS121 and is divided into a plurality of portions for separatephotoelectric conversion. In an exemplary embodiment, a hologram opticalelement (HOE) 161 diffracts and transmits the incident beam therebysplitting the incident beam into an error signal detection beam and aninformation detection beam. A condensing lens 163 condenses the beampassed through the HOE 161. The HOE 161 and the condensing lens 163 aredisposed on the optical path between the PBS 121 and the photodetector160.

In considering the difficulty in keeping the thickness error of theprotective layer 105 below ±3 μm during manufacturing of the opticaldisk 100, the optical pickup further includes a correcting unit 130 onthe optical path between the optical path changing unit and theobjective lens 150. The correcting unit 130 detects and correctsspherical aberration caused by thickness variations of the protectivelayer 105 and/or by optical disks having different thicknesses. Forexample, as illustrated in FIG. 6, the correcting unit 130 includes arelay lens 131 condensing the incident beam from the light source 110, acorrecting lens 135 arranged on the optical path between the relay lens131 and the objective lens 150, and an actuator 137 actuating thecorrecting lens 135 along the optical axis. Aberration caused bythickness variations of the optical disk 100 is detected and/orcorrected by actuating the correcting lens 135 along the optical axiswith the actuator 137.

In an exemplary embodiment, the actuator 137 is installed separatelyfrom the actuator 159 actuating the objective lens 150 along thefocusing and tracking direction to reduce a load applied to the actuator159 from the objective lens 150. Alternatively, the actuator 137 may beattached to the relay lens 131.

The relay lens 131 arranged on the optical path between the optical pathchanging unit 120 and the objective lens 150, condenses the parallelincident beam from the light source 110 to focus a focal point f infront of the objective lens 150. The correcting lens 135 arrangedbetween the focal point f and the objective lens 150 condenses theincident beam diverging after having been focused as the focal point fby the relay lens 131 such that the parallel incident beam go toward theobjective lens 150.

When moving the correcting lens 135 along the optical axis, the size ofthe beam incident on the first transmitting portion 151 of the objectivelens 150 varies. In particular, when the actuator 137 drives thecorrecting lens 135 along the optical axis toward the optical disk 100,the size of the incident beam of the objective lens 150 increases. Ifthe actuator 137 drives the correcting lens 135 along the optical axisaway from the optical disk 100, the size of the incident beam of theobjective lens 150 decreases. The focal position of the light spot canbe varied by adjusting the size of the incident beam of the objectivelens 150 so that spherical aberration caused by thickness variations ofthe optical disk and/or by adopting another optical disk having adifferent thickness can be corrected.

The correcting unit 130 can be applied to detect aberration due tothickness variations of the optical disk 100. In particular, whileactuating the correcting lens 135 along the optical axis, the light spotis focused on the recording surface 107 and the incident surface 103 ofthe optical disk 100 in succession. The incident beam reflected fromeach of the recording and incident surfaces 107 and 103 is received bythe photodetector 160 and focus error signals from the recording andincident surfaces 107 and 103 are detected. If the thickness of theprotective layer 105 of the optical disk 100 is beyond a predeterminedthickness, the incident beam focused on the recording surface 107includes defocus aberration, whereas the incident beam focused on theincident surface 103 includes no aberration. As a result, the twodefocus error signals have a relative offset and the thickness deviationof the optical disk 100 can be detected from the degree of offset. Atime lapse between the two focus error signals is calculated using acircuit (not shown) and the thickness of the optical disk 100 can becalculated by multiplying the time lapse by a scanning speed of thecorrecting lens 135. Thus, the thickness deviation of the adoptedoptical disk 100 may be detected.

Alternatively, in order to detect thickness deviations of the opticaldisk 100, a hologram optical element (HOE) (not shown) may be disposedbetween the light source 110 and the optical path changing unit 120. TheHOE diffracts and/or transmits a near-axis beam and a far-axis beam tobe focused on the recording surface 107 and the incident surface 103,respectively. Subsequently, while the correcting lens 135 or theobjective lens 150 is slightly actuated along the optical axis, thephotodetector 106 receives the near-axis beam and the far-axis beamreflected from the recording surface 107 and the incident surface 103. Atime lapse between two focus error signals is then detected from thenear-axis beam and the far-axis beam. The thickness deviation of theoptical disk 100 can be detected based on the time lapse. This techniquedetects the thickness deviation of the optical disk 100 and it may beapplied to detect comma aberration caused by tilting of the optical disk100.

The optical pickup adopting the correcting unit 130 as described abovedetects and/or corrects aberration caused by thickness deviations of theoptical disk 100 by actuating the correcting lens 135 along the opticalaxis. Thus, a light spot can be focused on the recording surface 107,irrespective of the thickness error of the optical disk 100, so thathigh quality record and reproduction signals can be obtained.

FIG. 7 illustrates an alternative embodiment of the optical pickup. Asingle correcting lens 135 is implemented as the correcting unit 130. Inthis embodiment, the objective lens 150 is designed to be suitable for adiverging incident beam. A parallel incident beam is condensed by thecorrecting lens 135 to focus a focal point f in front of the objectivelens 150 and then, the focused spot diverges towards the objective lens150. The size of the incident beam of the first transmitting portion 157of the objective lens 150 can be varied by actuating the correcting lens135 along the optical axis, such that the aberration caused by thicknessvariations of the optical disk 100 and/or by adopting another opticaldisk having a different thickness can be detected and/or corrected. SeeFIG. 6

The objective lens having the above configuration according to thepresent invention has the effect of shielding the near-axis beam andthus, a high NA of 0.8 or more can be achieved in a single lensconfiguration. The objective lens is able to sharply reduce the sidelobe components of a light spot so that a high-density light spot withultra high resolution may be focused on the optical disk. Therefore,when the objective lens is applied to optical pickups, microscopes,optical exposure apparatuses for manufacturing semiconductor devices,and mastering apparatuses for manufacturing optical disks, the opticalsystems of these apparatuses may be reduced.

The optical pickup adopts the single objective lens so that theconfiguration of the actuator is simple and optical aberration is keptat a low level. Further, an ordinary person skilled in the art willappreciate that the optical pickup provides for easy assembly andmanufacturing. The protective layer of the optical disk according to thepresent invention is thinner than 0.1-mm so that a thickness error inmanufacturing may be reduced to ±5 μm or less. Thus, the presentinvention makes it unnecessary to correct for spherical aberration dueto thickness deviations of the optical disk.

While this invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade thereto without departing from the spirit and scope of theinvention as defined by the appended claims.

1. An optical pickup, comprising: a light source emitting an incidentbeam; an optical path changing unit altering an optical path of theincident beam; an objective lens, including a first transmitting portionat a near-axis region of an optical axis of the objective lens todivergently transmit the incident beam, a second transmitting portionfacing the first transmitting portion to transmit the incident beam, afirst reflecting portion formed around the second transmitting portionto condense and reflect the incident beam from the first transmittingportion, and a second reflecting portion formed around the firsttransmitting portion to condense and reflect the incident beam from thefirst reflecting portion towards the second transmitting portion, so asto focus the incident beam from the light source and to form a lightspot on an optical disk; a photodetector receiving the beam reflectedfrom the optical disk and passed through the objective lens and theoptical path changing unit; and a detecting-correcting unit, including arelay lens to condense the incident beam and a correcting lens both ofwhich are arranged on the optical path between the optical path changingunit and the objective lens, the detecting-correcting unit actuating thecorrecting lens toward and from the relay lens upon a detection of athickness of the optical disk so as to correct an aberration caused bythickness variations of the optical disk.
 2. The optical pickup of claim1, wherein the objective lens is disposed on the optical path betweenthe optical path changing unit and the optical disk.
 3. The opticalpickup of claim 2, wherein the relay lens of the detecting correctingunit is closer to the light source than the correcting lens.
 4. Apickup, including a light source to emit an incident beam and an opticalpath changing unit to alter a path of the incident beam, the pickupcomprising: an objective lens, including a first transmitting portion ata near-axis region of an optical axis of the objective lens todivergently transmit the incident beam, a second transmitting portionfacing the first transmitting portion to transmit the incident beam, afirst reflecting portion formed around the second transmitting portionto condense and reflect the incident beam from the first transmittingportion, and a second reflecting portion formed around the firsttransmitting portion to condense and reflect the incident beam from thefirst reflecting portion towards the second transmitting portion, so asto focus the incident beam from the light source and to form a lightspot on an optical disk; a photodetector to receive the beam reflectedfrom the optical disk and passed through the objective lens and theoptical path changing unit; and a detecting-correcting unit, arranged onthe optical path between the optical path changing unit and theobjective lens, to correct an aberration of the incident beam caused bythickness variations of the optical disk upon a detection of a thicknessof the optical disk.